1. Field of Invention
The invention relates to systems and methods for aligning wafers or substrates. More specifically, the invention relates to systems and methods that align wafers or substrates that include microstructures or microcomponents.
2. Description of Related Art
During the arrangement of microstructures, a process may require that at least two substrates or wafers be aligned to each other so that the microstructures on the substrates are accurately aligned. The microstructures may be active or passive microstructures including electronic or micromechanical components. The substrates may include microcomponents, or microoptical or micro-electro-mechanical system (MEMS) devices. The MEMS devices may include accelerometers, gyroscopes, pressure sensors, and micromirrors. The substrates may be patterned with electronic devices such as transistors, inductors and sensors, or patterned with simple microstructures such as conducting lines or other geometries. The microoptical devices may include lasers, light-emitting-diodes, mirrors or lenses. Microchannels, fiber-clips and CLAW(ClawConnect™)-type contact springs may also be included on the substrates. Large substrates sometimes have to be aligned during an assembly, for example, of an image sensor array or display.
With conventional methods, most of the alignment of the substrates or wafers is performed optically with an x-y alignment stage. For example, the alignment may be performed by optical alignment with x-y stages and alignment marks. With this process, a calibration mask pattern and calibration wafer test pattern may include checkerboard-like arrays of mutually orthogonal (x-oriented and y-oriented) periodic gratings which are frequency matched to each other in the wafer plane, e.g., the image plane of the mask. The wafer test pattern may be carried by a die at the center of the wafer and by opposing radially displaced dies which align with the x-axis and the y-axis of the exposure tool, thereby enabling measurement of overlay air at various locations on the wafer.
MEMS fabrication technology is conventionally employed to align or join structures or substrates. Anisotropic etching of silicon is used to form optical benches. Moreover, Lithographie, Galvanoformung, Abformtechnik (LIGA) structures, (e.g., formed by X-ray lithography, electroplating, molding) are used to make connectors and to align optical components. These systems also align the components using x-y stages and alignment marks.
If an alignment process is required for substrates used with micro-optics or MEMS packaging, special tools or sophisticated optical systems are required for the alignment process. These areas may include assembly of displays or image sensors where the large area substrates must be aligned, e.g., the alignment of X-ray conversion screens or color phosphor screens to the pixels.
As shown in FIG. 1, and in order to fabricate, for example, an X-ray imager, a wafer 104 with the imager electronics, e.g., pixels 110, has to be aligned to a micro-patterned X-ray conversion screen 100. The alignment has to be precise so that each pixel 100 on the image sensor array matches a cell on the X-ray conversion screen 100. As shown in FIG. 2, CLAW springs 210 may be positioned on an upper substrate 202 and electrical contact pads 208 may be positioned on a lower substrate 204. After alignment of the two substrates 202 and 204, the upper substrate 202 may be lowered (and the lower substrate 204 may be adjusted in the x-y direction) so that an electrical connection is established between the CLAW springs 210 and the contact pads 208. The alignment accuracy may depend on the size of the CLAW spring 210 and the size of the contact pad 208. Using the conventional systems and methods, it is difficult to achieve an accurate alignment of the upper substrate 202 with the CLAW springs 210 and the lower substrate 204 with the contact pads 208 so that the electrical components establish a good connection.
Because a special optical alignment table may be required, an accurate alignment can be difficult to achieve. Furthermore, the equipment used for alignment using the conventional devices discussed above may be rather expensive and the alignment procedure time-consuming. In many instances, after the alignment of the substrates, the substrates have to be held in place with an adhesive. Thus, it becomes impossible to test and rework the devices after the adhesive cures. Some applications may require frequent exchange of parts which makes the optical alignment steps impractical.